In the context of the present invention with the term polyester elastomer is understood a block copolymer containing hard segments consisting of blocks of a high-melting crystalline or semi-crystalline aromatic or semi-aromatic polyester and soft segments consisting of blocks of a low-melting polymeric component. Such polyester elastomers are also known as TPE-E and copolyester block copolymers.
A tube typically comprises two end-parts and an elongated part between the two end-part. A tube comprising at least one stretched end-part obtained by mechanically stretching of at least a section of that end-part, is herein considered to be a stretched plastic tube. The elongated part and optionally one of the end-parts do not have to be subjected to mechanically stretching.
Basic requirements for plastic tubes used in automotive air-brake systems are that these tubes have a proper balance in flexibility and rigidity, are sufficient temperature resistance, and are resistant against the chemical environment present under the engine hood in cars. Conventionally, these tubes were made of polyamide 12. Amongst others due to the trend towards more compact cars, higher temperatures are generated in the automotive engine section, which made the use of polyamide 12 in this application critical. In view thereof, a search was made for better performing materials. As a replacement material, polyester elastomers have found their way into this application, because of their better performance (stiffness) at higher temperatures. The polyester elastomers used in plastic tubes for automotive air-brake systems are black, carbon black filled materials, like many other plastic materials used for other automotive under-the-bonnet applications. These polyester elastomer materials in general also show a good resistance against environmental influences like ozone. It has been observed however, that for more demanding situations, the ozone resistance of the airbrake tubes made of the said polyester elastomers is not sufficient, and the tubes can show environmental stress cracking when subjected to an environment comprising ozone. The said mechanical stress cracking can occur where the tubes are subjected to mechanical stretching prior to mounting the tubes to an engine block or to a brake system. Such a mechanical stretching occurs for example, when a barbed hose fitting or other type of hose fitting comprising thicker parts, is inserted in an end-part of the plastic tube. A barbed hose fitting is a hose fitting comprising backward projecting thickenings, intended to prevent easy extraction of the hose fitting from the tube. Fittings are needed for the mounting step. Barbed hose fittings can be produced and mounted economically and therefore are applied more and more. For that reason there is a need for plastic tubes, suitable for use in automotive air-brake systems, which are more ozone resistant.
The aim of the invention is therefore to provide a plastic tube, and more particular a stretched plastic tube, which has improved ozone resistance compared to the known plastic tubes obtainable from a polymer composition comprising a polyester elastomer.
This aim has been achieved with the stretched plastic tube according to the invention, wherein the stretched end-part comprises an outer surface layer area consisting of a first thermoplastic polymer composition (A) comprising a first copolyester elastomer (A-1) and 0-0.2 wt. % carbon black relative to the total weight of the thermoplastic polymer composition (A). Meanwhile, the stretched plastic tube comprises an inner surface layer consisting of a second thermoplastic polymer composition (B) comprising a second copolyester elastomer (B-1) and comprising more than 0.2 wt. % carbon black, relative to the total weight of the thermoplastic polymer composition (B).